1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method for forming an image using a multibeam system.
2. Description of the Related Art
A typical electrophotographic image forming apparatus first forms a latent image on a photosensitive element by exposing a charged photosensitive element to light emitted from a laser diode (LD) and then develops the latent image with developer. A typical LD emits one to four or, at most, eight laser beams from each semiconductor element. Surface-emitting lasers including vertical-cavity surface-emitting lasers (VCSELs) have been put to practical use and are available commercially. High-quality, high-speed image forming apparatuses using VCSELs have accordingly been provided.
A typical VCSEL emits about 40 laser beams from each chip. Therefore, if a VCSEL is used in an image forming apparatus to form a latent image, the image forming apparatus can perform high-quality, high-speed image formation. However, an adequate latent image cannot be formed just by replacing a laser device including an LD with a laser device including a VCSEL. The VCSEL emits multiple laser beams from a predetermined light-emitting area. It is necessary to control intensities of the laser beams to be emitted in order to form an adequate latent image. Because the intensities are integrated in the light-emitting area in the VCSEL, controlling the intensities of the laser beams (hereinafter, “intensity control”) is especially required to always form a high-quality latent image.
As the number of the laser beams increases, the intensity control takes longer if the intensity control is performed in the same manner. This eventually decreases the speed of image formation, which reduces the advantage of using the VCSEL. If some steps of the intensity control are skipped to increase the speed, a high-quality image cannot be formed.
Various technologies are offered to solve the problem. For example, Japanese Patent Application Laid-open No. 2007-021826 discloses an image forming apparatus and an optical writing device. The image forming apparatus or the optical writing device includes a light source unit that includes a plurality of light-emitting elements and an intensity detecting element that detects the intensity of light emitted from each of the light-emitting elements. The optical writing device includes volume resistances and sample/hold capacitors and uses those elements to perform intensity control. The number of the volume resistances is equal to the light beams. The intensities of the multiple laser beams can be controlled correctly with the method that is disclosed in Japanese Patent Application Laid-open No. 2007-021826. However, a larger control circuit is required to control the VCSELs. Moreover, it is necessary to adjust the same number of the volume resistances as emitted laser beams in order to perform intensity control, which decreases operational efficiency and requires additional maintenance.
In a typical color image forming apparatus, one LD or one VCSEL is allocated to one color. In contrast, in a scanning device disclosed in Japanese Patent Application Laid-open No. 2006-284822, two different-colored latent-images are formed using two beams that are split by a half-mirror prism from a single laser beam that is emitted from one VCSEL. The manner of exposure using the split beams is called, hereinafter, “split-beam exposure”. The split-beam exposure decreases the number of the required VCSELs by half, which reduces the manufacturing cost and occurrence of failures related to the light sources.
If the split-beam exposure disclosed in Japanese Patent Application Laid-open No. 2006-284822 is used, the number of the volume resistances and the number of the sample/hold capacitors become double, which requires a much larger control circuit. In the split-beam exposure, as described above, the latent images are formed on the two photosensitive elements for different colors using the laser beam emitted from a single VCSEL. Because, after splitting by the half-mirror prism, the split beams travel along different optical paths, the transmittances of the split beams are different from each other. Therefore, it is necessary to correct the intensity of the laser beam that is emitted from the single VCSEL using different correction values.